1. Technical Field
The invention relates to a fixing device for heating and pressurizing a toner image, which is formed by selectively depositing toner on a latent image based on an electrostatic potential difference and transferred onto a recording medium, so as to form a fixed image and an image forming apparatus using the fixing device.
2. Description of the Related Art
In an electrophotographic image forming apparatus, image light is applied to a charged surface of a photosensitive drum. Thereby, an electrostatic latent image is formed. Then, toner is electrostatically transferred to the electrostatic latent image. Thereby, a toner image is formed on the photosensitive drum. Then, the toner image is transferred directly or through an intermediate transfer body to a recording medium and is fixed onto the recording medium.
The following fixing device for fixing an unfixed toner image onto a recording medium has been widely used. That is, the fixing device includes a heating roller, which serves as a heating rotation body and has an endless peripheral surface moving circularly; and a pressurization roller, which serves as a pressurization member for pressing a toner image against the peripheral surface. In this fixing device, the heating roller and the pressurization roller are in pressure-contact with each other and a recording medium on which the toner image is carried is inserted into a pressure-contact portion (nip portion). Accordingly, the recording medium passes through the pressure-contact portion as the heating roller rotates and then, the unfixed toner image is heated and is fixed.
The heating roller for use in such a fixing device often has a hollow cored bar made of aluminum and a heater supported inside the cored bar. A surface of the heating roller is coated with a fluorocarbon resin to enhance releasability. The pressurization roller includes an elastic layer having heat resistance, such as silicone rubber, on a metal cored bar. Since toner transferred to the heating roller, that is, offset toner may be transferred to the back of the recording medium, which makes the back of a recording medium dirty. Therefore, it has been proposed that the pressurization roller is provided with a surface layer based on a fluorocarbon resin on which toner is hard to deposit. For example, the peripheral surface of the pressurization roller is covered with a PFA resin layer shaped like a thin tube.
In the fixing device with the surface of the pressurization roller thus covered with a fluorocarbon resin based material, the surface has high insulation properties and thus the surface of the pressurization roller is charged by friction when a recording medium on which an unfixed toner image is carried passes through the nip portion. Generally, the fluorocarbon resin is charged to the negative polarity by friction between the fluorocarbon resin and paper. When the negative potential on the surface of the pressurization roller becomes high, the negative polarity toner on the recording medium electrically repels between the recording medium and the surface of the pressurization roller. As a result, electrostatic offset, that is, transferring of the toner to the surface of the heating roller easily occurs.